Salesforce userinfo's top-level id field embeds the MyDomain host as the Identity URL

The Salesforce userinfo response (<instance_url>/services/oauth2/userinfo) returns a top-level id field of the form https://MYDOMAINNAME.my.salesforce.com/id/<orgId>/<userId>. The MyDomain host is parseable directly from this URL, and is also embedded in every entry of the urls.* map. This means consumers don't need to thread instance_url from the original token-grant response into the identity-fetch step, userinfo alone is sufficient to derive the host. Sandbox vs production orgs surface in the host (e.g. *.sandbox.my.salesforce.com).

To get a Salesforce instance host alongside identity, parse the host out of the userinfo id URL rather than persisting instance_url from the OAuth token-grant response separately.